Tuesday, August 21, 2012

Q&A: Compare Series vs. Parallel Fan-Powered Terminal Units

There are two types of fan-powered
terminal units - series and parallel. Every manufacturer offers both types and
special variations such as low profile and quiet units. Although the type of
unit to use is often up to personal preference, there are distinct differences:

Series fan-powered terminals have
fans that must run throughout the occupied mode in order to deliver ventilation
air to the zone:

These
units act as boosters for the air handler because their fans move the
air the rest of the way to the zone. This allows the air handler to run at
system pressure far lower than other types of terminal units require. The
typical system pressure supplying series fan boxes is 0.50 IN WG.

Since
the fan runs continuously during occupied periods, they provide constant
air motion and more air changes than other types of terminal units.

The
continuous operation of the fan results in relatively constant sound
levels, unlike other types of terminal units that vary air volumes and/or
cycle fans.

Parallel fan-powered terminals have
fans that only switch on during the heating mode to pull warm return air from
the ceiling plenum:

Since
the unit fan is off during the cooling mode, the box acts like a single
duct VAV and simply varies airflow from the air handler to maintain room
temperature. Typical system pressures are between 1.00 and 1.50 IN WG.

Some
engineers do not specify parallel fan units because the fan cycling is
often noticeable to occupants.

Parallel
fan units must include a backdraft damper to prevent primary air from
leaking back through the blower into the ceiling plenum. Leakage around
the backdraft damper can be an issue and could be considerable when
downstream pressure requirements are greater.

An ASHRAE research project (RP-1292)
completed in 2007 was conducted to determine which type of fan-powered terminal
used the least energy from a whole building perspective. The report said that
either unit could be equally efficient when properly sized and applied. This
original report only included units with standard PSC fan motors. A subsequent
addendum to the report, paid for by a consortium of interested parties, took
the newer ECM technology into account in the same energy model. It gave more of
an advantage to the series fan units.